Process for making a spark plug



Nov. 7, 1967 M. J. ANTUNES 3,350,759

PROCESS FOR MAKING A SPARK PLUG Filed Aug. 15, 1963 INVENTOR. MANOELJOSE ANTUNES ATTORNEYS 2 Sheets-Sheet 1 Nov. 7, 1967 Filed Aug. 15. 1963M. J. ANTUNES PROCESS FOR MAKING A SPARK PLUG FIGS 2 Sheets-Sheet 2 92INVENTOR. MANOEL JOSE ANTUNES 9;; JAM

ATTORNEYS United States Patent ()fifice 3,35%,759 Patented Nov. 7, 19673,350,759 PROCESS FOR MAKENG A SPARK PLUG Manuel .los Antunes, Rua FelixEacheeo 93,

Rio de Janeiro, Brazil Filed Aug. 15, 1963, Ser. No. 362,385 2 Claims.(Ci. 29-25.12)

This invention relates generally to spark plugs for internal combustionengines, and more particularly to an improved process for making suchspark plugs and the product thereof.

Presently the trend in manufacture of high combustion engines is toproduce engines with higher and higher coinpression ratios. Theserequire a perfect sealing of the central electrode of the spark plugsused therein. To obtain an adequate seal that will prevent high pressureleakage, the spark plug makers have been forced to produce the centerelectrode, the inner conductor, and the terminal of the spark plug inseparate or multi-step operations and to seal these parts to the bore ofthe spark plug insulator by a complex glass sealing process as forexample, those described in Patents 2,449,403 and 2,459,852. Thesemethods of fabricating and sealing the central electrode are expensivebecause of the number of steps required and the materials used.

It is a primary object of the present invention to overcome the abovestated disadvantages of the conventional methods of sealing spark plugsand, in particular, to utilize the metal from which the inner conductorof a spark plug is formed to seal itself during its formation, thusavoiding the need for additional sealing materials and additionalprocess steps to obtain an adequate seal.

It is another important object of the present invention to provide animproved process for fabricating spark plugs in which the innerconductor alone, or the inner conductor and exposed terminal, are castby injecting molten material at high pressure into the bore of theinsulator, the solidification of the metal resulting in a contraction,or shrinkage, which tightly bonds the inner conductor to a spiral orhelical channel in the insulator bore and thus provides a pressure-tightseal for the center electrode and inner conductor.

It is a further object of the invention to provide an improved processof fabricating spark plugs as briefly outlined above, wherein the innerconductor and exposed center terminal are integrally formed in onepiece, and the multi-step conventional processes for forming and sealingthe central electrode, inner conductor and central terminal are replacedby a single, elficient, and inexpensive operation.

Another object of the invention is to provide an improved spark plughaving an integrally united central electrode, inner conductor'andcentral terminal well sealed to the insulator and formed as a product ofthe process briefly described above.

Still another object of the invention is to provide an improved processfor forming a complete spark plug in which the inner conductor, centralterminal and outer shell are simultaneously formed by high pressureinjection molding in one operation.

The novel features that are considered characteristic of the inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and its method ofoperation, together with additional objects and advantages thereof, willbest be understood from the following description of specificembodiments when read in connection with the accompanying drawings,wherein like reference characters indicate like parts throughout theseveral figures and in which:

FIG. 1 is a longitudinal axial section of an insulator used in theimproved process according to the invention;

FIG. 2 is a similar axial section showing the insulator capped by a moldand supported for injection casting of the inner conductor and centralterminal;

FIG. 3 is an axial section similar to FIG. 1 showing the insulatorremoved from the mold after completion of the molding step which formsthe inner conductor and central terminal;

FIG. 4 is an enlarged fragmentary sectional view of a portion of theinsulator after the inner conductor has been cast;

FIG. 5 is a perspective view of a central electrode used in the processaccording to the invention;

FIG. 6 is an axial sectional View similar to FIG. 1 of a modifiedinsulator;

FIG. 7 is a fragmentary sectional view similar to FIG. 2 showing amodified mold;

FIG. 8 is a sectional view similar to FIG. 2 but of a modified mold forsimultaneously casting an inner conductor and an outer shell about aninsulator to form a complete spark plug;

FIG. 9 is an elevation partly in axial. section of a finished spark plugmolded in the apparatus of FIG. 9; and

FIGS. 10 and 11 are views respectively similar to FIGS. 8 and 9 butshowing modified molding apparatus and the completed spark plug obtainedtherein.

Referring now more specifically to FIGS. 1-5 of the drawing whichillustrate the steps of the improved process, FIG. 1 shows a tubularinsulator 10 which may be of any conventional shape and formed ofporcelain, ceramics or other conventional spark plug insulator material.The insulator is provided with a non-conventional central bore having asmaller diameter in the lower portion 12 than in the upper portion 14,the two portions passing completely through the insulator axiallythereof and being separated by shoulder 16. The upper portion of thebore during fabrication is formed with a spiral channel, or groove 18,for substantially its complete length and which, as illustrated in FIG.4, may take the form of a threadlike groove having lower and upper,inclined, plane sides 18a and 18b.

In the improved process it is preferred to utilize a conventional typecenter electrode shown at 26 (FIG. 5) but in which the head 22 isflattened, or pinched, so as to project a portion of the materialdiametrically of the electrode pin 20. The pin 20 is then inserted inthe bore portion 12 of the insulator from the top, either by hand orautomatically by machine, until the head 22 engages and rests on theshoulder 16 (see FIG. 2). The wall of the bore portion 12 preferablymakes a close fit about the electrode 20, and when the electrode issupported by its head on shoulder 16 the lower end of the electrodeprojects slightly from the bottom of the insulator. As thus supported,the electrode is virtually centered. and an annular space surrounds thehead 22 in the enlarged bore portion 14 for reception of metal flashduring the injection of molten metal.

The assembled insulator and electrode 20 are then placed or moved bymachine for support on a base structure 24 having a cavity 26 whichclosely approximates the external shape of the insulator but leaves anopening at the lower end for the protruding electrode 20 to be receivedwithout touching the base under pressure of the casting operation. Thebase cavity 26 is provided with a shoulder 28 which seats the lower endof the insulator to restrain the insulator from downward motion underthe pressure of the casting or molding step to be described.

The upper portion of the insulator 10 is then surrounded by the moldhalves 30 and 32 having conduits 34 and 36 respectively, for passage ofa coolant such as Water at low temperature. The assembled mold halvesprovide a mold cavity 38 shaped in the form of a conventional spark plugterminal, such as that illustrated at a 44, FIG. 3, which has an exposedbulbous head with a cylindrical central portion and reversed conicalupper and lower adjacent surfaces on a cylindrical neck and base with anenlarged flange 46 defined by the portion 48 of the mold cavity 38.

To form the inner conductor and terminal of the spark plug, molten metalis introduced into the die cavity 38 through the opening 39 under highpressure. The molten metal completely fills the bore portion 14 andthread groove 18, flashing around the head 22 of central electrode toform a primary seal at the shoulder 16 of the bore. The supply of moltenmetal is instantaneously cut off and the metal allowed to cool andsolidify aided by water or other coolant passing through the passages 34and 36 in the directions of the arrows shown in FIG. 2.

During solidification, the metal shrinks and because of the differencein coefiicient of linear expansion of said metal as compared to that ofthe insulator 10, the metal will shrink more than the insulator and tendto separate considerably from the wall of bore 14 at its upper and lowerends but less at its center. In this way the solidified inner conductor42, FIG. 3, will have shrunk inwardly from its ends toward its center tointimately bond to the surface interstices of the groove in the bore ofthe insulator. As illustrated in FIG. 4, the central portion ofconductor 42 does not shrink materially and tightly bonds to both lowerand upper plane sides 18a and 18b of the thread-like channel in thecentral area 10a of the insulator. In shrinking, the metal of the upperportion of the conductor 42, in the insulator area 10b, separatesslightly, as shown in exaggerated manner, from the upper Walls 18b ofthe thread groove, but shrinks tightly against the lower walls 18a toform a very tight seal. In the lower portion 100 of the insulator,corresponding to the lower portion of bore 14, the metal of the innerconductor 42 contracts upwardly into tight engagement with the upperplane surfaces 18b of the thread groove. Thus at least some portion ofeach increment of the entire length of the thread groove in bore portion14 is tightly sealed by the solidified metal.

Upon cooling of the metal in the bore of the insulator, the latter maybe removed from the support 24 and the mold halves and 32 to yield theproduct, illustrated in FIG. 3, in which the inner conductor 42 andexternal terminal 44 are integrally united to the center electrode 20.The shoulder 46, formed in the mold cavity portion 40, is tightly shrunkand solidified against the upper edge of the insulator to seal the boreat that point while a primary seal is obtained by the flash metalinjected around the head 22 of the electrode in the area of the boreshoulder 16. In addition, the above described shrinkage in twodirections in the spiral channel 18 tightly seals the inner conductor 42for the entire length of the channel in bore portion 14, so that thesingle injection operation and the use of only the metal of the innerconductor, without other sealing materials, serves to effectively sealthe electrode, the inner conductor and the terminal against highpressure leakage through the bore of the insulator.

In molding the inner electrode according to the above described processany suitable metal may be utilized, for example, substantially purealuminium or nickel, or alloys thereof, zinc alloys, steel, and others.The molten metal is preferably injected at very high pressure, asuitable pressure having been found to be about 50008000 p.s.i. whenforming the inner conductor to a center electrode having a diameter ofapproximately /8 of an inch and a length of about 1 inch. The shrinkageand resultant bonding of the cast metal is due to the difference in thecoefficient of thermal expansion of the porcelain insulator from that ofthe casting metal. The coefiicients of thermal expansion (linearexpansion per 1 F.) for the named metals are approximately: aluminum,14.2 10 nickel, 7 10 carbon steel, 5.57.2 10 and porcelain 2 10 intemperature. The ratio of linear contraction of any of the above metalsto that of porcelain is adequate to provide for considerable shrinkageand to thereby affect the tight seal and bonding to the spiral groove inthe wall of the bore of the insulator.

The injection of the metal under high pressure is nearly instantaneous.The metal of inner conductor 42 is allowed to cool and solidifies inapproximately 10-15 seconds at which time the mold may be removed. Thewater cooling of the mold aids in conducting heat away from the injectedmetal and materially increases the speed of its solidification. If apoor grade of porcelain or ceramics is used, making the insulator proneto fracture under the shock of injection of metal at fusion temperature,the insulator may be pre-heated before injection of the metal into thebore of the insulator.

In FIG. 6 is shown a modified insulator 10 in which the spiral threadgroove 18 is replaced by a spiral groove 48 having a semicircular orellipsoidal cross section. In such a spiral groove the solidifying metalwill again contract toward the lower or upper sides of the groove and,because of the curved shape of the groove, a complete locking of thesolidifying metal to the wall will result, further improving the seal.

In FIG. 7 is disclosed a modified embodiment of the mold in which themold halves 50 and 52 are provided with a cavity 54 shaped to form aterminal in the form of a threaded stud, the bottom portion 56 of thecavity having a shape which will form a nut-like base on said terminalstud. The mold cavity has an opening 58 for entrance of the injectionmaterial.

While the process for making the spark plug as thus far described isconcerned solely with the formation of the integral electrode, conductorand terminal, it is also contemplated that simultaneously with theformation of these portions of the spark plug, an outer metal shellincluding a cylindrical metal sleeve and an outer electrode may beinjection molded at the same time. US. Patent No. 3,077,649 discloses aprocess for injection molding of the outer shell and outer terminal, andthe process fully described therein, which will not be repeated here, iscontemplated to be combined with the process for forming the centerelectrode and center terminal, above described, by a suitablemodification of the mold parts so that both the internal and externalconductor (shell) of the spark plug may be simultaneously injectionmolded in and about the insulator in one operation.

In FIG. 8 is shown one embodiment of mold apparatus capable of combiningthe injection molding of the inter nal conductor and outer shell in oneoperation. The mold halves 60, 62 are similar to mold parts 30, 32,above described, except that they are lengthened to surround the entireinsulator 74 as a core. Additional and connected cavities referenced asthe shell cavity 64, are formed in mold parts 62, 64 suitable to formshell 80, FIG. 9, having a lower portion with external threads andtightly shrunk about the insulator ribs 76. The mold parts are supportedon base 66 having a central cavity 68 with outwardly flared walls whichclosely support the lower portion of insulator 74. A metal sleeve 70also supports the insulator as a central core in mold cavity 64, while ametal strip 72 is supported adjacent the sleeve in a channel in base 66.Parts 70 and 72 are similar to the corresponding sleeve and outerelectrode described in Patent 3,077,649. When molten aluminum, or othermetal, is injected into mold cavities 38 and 64 simultaneously throughopening 39 and a second opening, not shown, the inner conductor 42,central terminal 44, outer shell and outer electrode 72, FIG. 9, aresimultaneously formed and in the completed electrode shell 80, sleeve 70and outer terminal 72 are integrally united.

FIG. 10 illustrates another modified mold apparatus wherein mold halves60, 62 are again used substantially without change but the base 84 isprovided with a slot 86 positioned to cast the outer terminal of thespark plug directly without using sleeve 70 and metal strip 72 of FIG.8. The resultant spark plug, cast in the FIG. 10

apparatus, is shown in FIG. 11 as having outer terminal 92 moldedintegrally With sleeve 90 and simultaneously with inner conductor 42 andcentral terminal 44. The outer electrodes 72, 92, FIGS. 9 and 11 may bebent to underlie the central electrode, as shown in broken lines, afterthe spark plug is removed from the mold.

It is apparent from the above that many changes in the size, shape andform of the structure and parts fabricated by the process may be made,as well as in the materials and molds used for casting of the describedparts. The casting is not limited to a vertical direction and the moldmay be cooled by air fins rather than water ducts. Accordingly, it iscontemplated that the invention shall be limited only by the scope andspirit of the appended claims.

What is claimed is:

1. A method of making a spark plug having a terminal at its upper end,comprising the steps of:

fabricating an insulator having a central bore that is smooth at itslower end and having a spiral channel in its wall for a substantialportion of its length and terminating at its upper end, placing anelectrode in the unchanneled portion of the bore, and 5 injecting moltenmaterial under high pressure into said bore to form an inner conductor,through a set of dies including a mold cavity which forms the terminalprotruding from the upper end of said bore. 2. A method of making aspark plug according to 10 claim 1 including water cooling said dies aspart of the operation.

References Cited UNITED STATES PATENTS 15 1,347,367 7/ 1920 Gerbaud22202 1,362,773 12/1920 Brewster 22-202 2,400,917 5/ 1946 Corbin 22-202X 3,077,649 2/1963 Muniz 22202 20 WILLIAM I. BROOKS, Primary Examiner.

1. A METHOD OF MAKING A SPARK PLUG HAVING A TERMINAL AT ITS UPPER END,COMPRISING THE STEPS OF: FABRICATING AN INSULATOR HAVING A CENTRAL BORETHAT IS SMOOTH AT ITS LOWER END AND HAVING A SPIRAL CHANNEL IN ITS WALLFOR A SUBSTANTIAL PORTION OF ITS LENGTH AND TERMINATING AT ITS UPPEREND, PLACING AN ELECTRODE IN THE UNCHANNELED PORTION OF THE BORE, ANDINJECTING MOLTEN MATERIAL UNDER HIGH PRESSURE INTO SAID BORE TO FORM ANINNER CONDUCTOR, THROUGH A SET OF DIES INCLUDING A MOLD CAVITY WHICHFORMS THE TERMINAL PROTRUDING FROM THE UPPER END OF SAID BORE.